Page 3, What makes a trailer stable?

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What makes a good Trailer?

Trailer Design
Trailer Design
for Stability

Links for further reading:
1. Introduction.
2. Trailer Strength.
3. Stability.
4. Versatility.
Choosing the Right
Utility Trailer.

More Articles . . .

Page 3: Trailer Stability
Why are some trailers stable while others wander all over the road?
There are dozens of factors related to towing stability. No one factor is an absolute for making a trailer stable or not, but they all have an effect. Here are a few of the most common things to look for:

An extreme example of poor load distribution.

Load distribution is the most common cause of stability concerns -- and the easiest to correct. The trailer load should be reasonably distributed from side to side with at least 10% of the trailer weight on the hitch. With typical trailers, more weight on the hitch is better for stability. 15% is usually a good number.
For the tow vehicle, "motor boating" is a stability concern. With a large weight applied at the hitch in the rear of the vehicle, the front of the vehicle can begin to rise. When this condition is significant, it will adversely effect stability, driveability and control because weight is lifted off the steering wheels. To correct this problem, load distributing hitches can be used to level the trailer with respect to the tow vehicle. A sample is shown below. These hitches can make a big difference -- especially when trailer size and/or weight is near or exceeds that of the tow vehicle.
Note: Before using a load distributing hitch, make sure your coupler can handle it.

A load distributing hitch example from reese-hitches.com
Usually if a Damper (like a big shock absorber between the trailer and the tow vehicle) is needed there are other, perhaps fundamental, issues that should be addressed. A trailer properly configured and properly loaded should not need a damper -- though in a pinch, they can help. (Note: One is shown in the photo above.)
Trailer length and length with respect to width is another factor. Longer trailers typically tow better -- or more accurately, a greater separation between the rear most wheels of the tow vehicle and the fore most wheels of the trailer. A long tongue as discussed in the Strength section can help in facilitating separation. The point here is really the distance with respect to the width. Look at the width of the trailer with respect to the tow vehicle and with respect to the axle separation. The wider the trailer, the more it will benefit from length.
Another length factor is the distance between the rear wheels of the tow vehicle and the hitch point. When this distance is shorter, the trailer can't "wag" the tow vehicle as much. Fifth wheel trailers are a good example. Extend-a-hitch bars that push the hitch point way back are a bad example.

A couple of Axle and Wheel related issues:

First, use a trailer axle - an axle made for a trailer. The wheels on a proper trailer axle have just a little camber to them -- which greatly enhances tracking.
Axle alignment with respect to the pull direction is very important. The axle(s) should be perpendicular to the direction of pull -- and perhaps more important, if more than one axle is used, they must be parallel. (This condition is easy to see from behind on the highway -- because the trailer follows behind and a little to the side of the tow vehicle.)
Wheels and tires should be in good condition. I can't tell you how many wobbly (bent) rims I have seen on trailers going down the highway. The wobbly may come from a bent rim, or from a wheel that is not attached properly. Either way, it causes problems with stability -- and safety.
Tires should be properly inflated. Inflation pressure will vary depending on conditions, however. When the trailer is to be pulled a long distance empty, or without significant load, tire pressure should be reduced -- perhaps to 10 or 15 psi. This allows the tires to soak up more of the road inconsistencies, and the tires become a damper to keep the trailer from bouncing around. When the trailer is loaded, the inflation pressure should be increased per the tire manufacturers recommendations.
Though not a specific stability concern (until there is a blow-out) Dry Rot on Trailer Tires is one of the most common failures. It's a horrible way to interrupt your trip. Check your tires before traveling.

Brakes and calibrating the brakes to the tow vehicle is worth a long discussion (which we won't give here). Just note that the brakes need to be applied appropriately for the load and trailer size. This is to be less of an issue with electric brakes than with surge type, but it applies to both. Numerous issues with setting and calibrating surge brakes have caused several states to consider banning their use. Check local regulations before buying a trailer with surge type brakes.
Dynamic loads can have a significant effect on stability. For instance, if a large, heavy pipe is carried untied, it will roll from side to side or front to back with motion of the trailer. This constantly changing load makes the trailer move. If the dynamic load is small compared to the weight of the trailer, the effect may not be important, but if the moving load is a large, the effect can be quite important. Live loads like animals can also have this effect. In general, the load should be secured as much as possible to accommodate stability.
Center of gravity is another important consideration. In general, a lower center of gravity helps stability. I once saw a trailer that tipped over around a corner because the CG was too high for the conditions.
Aerodynamic effects, caused by the load, side winds or other vehicles can contribute to stability issues. Typically high profile and larger trailers like campers suffer from this more, but be careful to load a utility trailer aerodynamically even. Things like sharp corners on the trailer contribute (in a minor way) to aerodynamic stability.
Along with aerodynamics, trailer attitude gives another minor effect. Trailer attitude is the relationship of the trailer with respect to the ground and tow vehicle -- basically, the trailer should be somewhat level (not with the butt dragging the ground or the tongue extra low).
One final item is the strength or rigidity of the trailer itself. If the trailer is flimsy it will twist and bend with the dynamics -- especially if the load is also dynamic. Just one more reason to choose a sufficiently strong trailer as discussed on Page 2: Trailer Strength.

Summary:
There are numerous items that effect the stability and safety of a trailer - all need some consideration, but a few are most important. Items of construction include wheel and axle alignment, axle placement, brakes, calibration, and overall strength. Effects to be considered with each use include trailer attitude with respect to the ground and the tow vehicle, tire condition and inflation pressure, and (perhaps most important) load distribution and load securing. By considering, adjusting and making sure these things are correct, the trailer will tow in a stable, predictable and enjoyable manner.

Next: Versatility - A look at trailer options.

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